Synergistic use of limb and nadir observations for studying ozone stratospheric intrusions in the Himalayan region.

Tropospheric ozone plays a key role in atmospheric chemistry, dynamics, and radiation, influencing both climate and air quality. Among its natural sources, stratosphere–troposphere exchange (STE) processes significantly contribute to ozone variability in the upper troposphere, especially over complex regions such as the Himalayas, where orography and meteorological conditions favor stratospheric intrusions. However, observational limitations - particularly in vertical resolution and spatial coverage - restrict a detailed characterization of these events. This thesis wants to address these challenges by developing and validating a novel ozone dataset for 2008–2011 obtained through Complete Data Fusion (CDF) of limb observations from MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) and nadir observations from IASI (Infrared Atmospheric Sounding Interferometer). The fusion method exploits the complementary strengths of the two instruments, enabling the transfer of high-quality limb information into the troposphere, where limb measurements are not available, thus completing and enhancing the nadir profiles. The resulting product exhibits improved vertical information content and reduced uncertainties, particularly in the Upper Troposphere–Lower Stratosphere (UTLS), and provides more complete ozone profiles than either dataset alone. The second part of the work presents an initial exploration of stratospheric intrusions over the Himalayan region. A case study (28 January–2 February 2008) was analyzed using satellite, model, and reanalysis data to visualize the intrusion and characterize the associated ozone profiles. An unsupervised classification approach based on Principal Component Analysis and k-means clustering was applied to distinguish profiles representative of intrusion conditions, demonstrating the potential of the fused dataset for objective identification. Overall, this work lays the foundation for future large-scale and climatological investigations of stratospheric intrusions and their role in STE processes, highlighting the value of limb–nadir fusion for advancing atmospheric composition studies.